Schroerlucke Samuel R, Steklov Nikolai, Mundis Gregory M, Marino James F, Akbarnia Behrooz A, Eastlack Robert K
Tabor Orthopedics, Division of MSK Group PC, Memphis, TN, USA.
Clin Orthop Relat Res. 2014 Sep;472(9):2826-32. doi: 10.1007/s11999-014-3711-x. Epub 2014 Jun 12.
Minimally invasive spinal fusions frequently require placement of pedicle screws through small incisions with limited visualization. Polyaxial pedicle screws are favored due to the difficulty of rod insertion with fixed monoaxial screws. Recently, a novel monoplanar screw became available that is mobile in the coronal plane to ease rod insertion but fixed in the sagittal plane to eliminate head slippage during flexion loads; however, the strength of this screw has not been established relative to other available screw designs.
QUESTIONS/PURPOSES: We compared the static and dynamic load to failure in polyaxial, monoaxial, and monoplanar pedicle screws.
Six different manufacturers' screws (42 total) were tested in three categories (polyaxial, n = 4; monoaxial, n = 1; monopolar, n = 1) utilizing titanium rods. An additional test was performed using cobalt-chromium rods with the monopolar screws only. Screws were embedded into polyethylene blocks and rods were attached using the manufacturers' specifications. Static and dynamic testing was performed. Dynamic testing began at 80% of static yield strength at 1 Hz for 50,000 cycles.
In static testing, monoaxial and monoplanar screws sustained higher loads than all polyaxial screw designs (range, 37%-425% higher; p < 0.001). The polyaxial screws failed at the head-screw interface, while the monoaxial and monoplanar screws failed by rod breakage in the static test. The dynamic loads to failure were greater with the monoplanar and monoaxial screws than with the polyaxial screws (range, 35%-560% higher; p < 0.001). With dynamic testing, polyaxial screws failed via screw-head slippage between 40% and 95% of static yield strength, while failures in monoaxial and monoplanar screws resulted from either screw shaft or rod breakage.
All polyaxial screws failed at the screw-head interface in static and dynamic testing and at lower values than monoaxial/monoplanar screw designs. Monoplanar and monoaxial screws failed at forces well above expected in vivo values; this was not the case for most polyaxial screws.
Polyaxial screw heads slip on the screw shank at lower values than monoaxial or monoplanar screws, and this results in angular change between the rod and pedicle screw, which could cause loss of segmental lordosis. The novel monoplanar screw used in this study may combine ease of rod placement with sagittal plane strength.
微创脊柱融合术常常需要通过小切口置入椎弓根螺钉，视野受限。由于使用固定单轴螺钉插入棒材困难，多轴椎弓根螺钉更受青睐。最近，一种新型单平面螺钉问世，它在冠状面可移动以方便棒材插入，但在矢状面固定以消除屈曲负荷下的头部滑移；然而，相对于其他现有螺钉设计，这种螺钉的强度尚未确定。
问题/目的：我们比较了多轴、单轴和单平面椎弓根螺钉的静态和动态破坏载荷。
使用钛棒对六个不同制造商的螺钉（共42枚）进行三类测试（多轴，n = 4；单轴，n = 1；单极，n = 1）。仅对单极螺钉使用钴铬棒进行了额外测试。将螺钉嵌入聚乙烯块中，并按照制造商的规格连接棒材。进行静态和动态测试。动态测试在1Hz频率下从静态屈服强度的80%开始，持续50000次循环。
在静态测试中，单轴和单平面螺钉承受的载荷高于所有多轴螺钉设计（范围，高37% - 425%；p < 0.001）。在静态测试中，多轴螺钉在螺钉头部界面处失效，而单轴和单平面螺钉因棒材断裂而失效。单平面和单轴螺钉的动态破坏载荷大于多轴螺钉（范围，高35% - 560%；p < 0.001）。在动态测试中，多轴螺钉在静态屈服强度的40%至95%之间因螺钉头部滑移而失效，而单轴和单平面螺钉的失效是由于螺钉轴或棒材断裂。
在静态和动态测试中，所有多轴螺钉均在螺钉头部界面处失效，且失效载荷低于单轴/单平面螺钉设计。单平面和单轴螺钉在远高于预期体内值的力作用下失效；大多数多轴螺钉并非如此。
多轴螺钉头部在螺钉杆上的滑移值低于单轴或单平面螺钉，这会导致棒材与椎弓根螺钉之间的角度变化，进而可能导致节段性前凸丧失。本研究中使用的新型单平面螺钉可能将棒材放置的便利性与矢状面强度相结合。
